Our invention relates to the conversion of electric signals from analog to digital form. More specifically, our invention pertains to a method of, and a system for, digitizing an audio or like data signal, with the addition of analog dither (an artificially created white noise signal) to the analog data signal and the subsequent removal of the dither from the digitized data signal, with a view to the reduction of noise and distortion. Still more specifically, our invention concerns improvements in such a method and system whereby the analog dither to be added to the data signal is suppressed or reduced in level, as required, for the analog to digital conversion of the data signal with a greater dynamic range than heretofore under the same hardware limitations.
The pulse code modulation (PCM) or digital processing of audio signals has become, or is becoming, the mainstream of high fidelity sound recording and reproduction with the advent and ever increasing commercial acceptance of compact discs (CDs). However, some problems remain unsolved, or not perfectly solved, in the art of digital sound processing. One of these is the "quantization noise", that is, the differences between the samples of the music wave and the quantized values of the samples. The quantization noise becomes particularly pronounced, distorting the reproduced sound as higher harmonics, when the input signal level is low and there are a relatively small number of quantization steps. Even when the input signal level is high, the quantization noise will distort the signal if it changes slowly.
We are aware of a conventional solution to this quantization noise problem. The solution involves the use of dither for turning the quantization noise into white noise which hardly affects the appreciation of the reproduced sound. An analog dither signal is superposed on the audio signal prior to its analog to digital conversion and, following the conversion, is removed from the digitized audio signal. This technique is discussed extensively in the article entitled "The Application of Large Amplitude Dither to the Quantization of Wide Range Audio Signals" by Yoshio Yamasaki in The Journal of the Acoustical Society of Japan, Vol. 39, No. 7, published 1983. Japanese Laid Open Patent Application No. 50-68258 also discloses the same technique.
We have found that this known solution has a weakness. As dither is added to the analog audio or other data signal, the total magnitude of the resulting data and dither signal may exceed the capacity (maximum allowable input voltage) of the analog to digital (A/D) converter used for the conversion of the data and dither signal, particularly when the data signal level is high. There might be contemplated the use of an A/D converter of sufficiently large capacity to accept the expected maximum magnitude of the combined data and dither signal. Such hardware is very expensive and so impractical.